Mark Foster is a main character on ABC family comedy Step by Step. He is portrayed by Christopher Castile.
Character[]
Carol's only son and the youngest child in the family, who is depicted as a nerd, and is into computers and academics. Later on, he matures to be "a normal teenager" and has friends and a girlfriend, while still a high academic achiever. He seems to be somewhat similar to Steve Urkel, although Mark has not proposed as many outlandish inventions. In a crossover with Family Matters, Mark works with Steve Urkel on a science project for a national competition.